using google sitemaps with dynamic content

Hello,

If your site is dynamic, and changes every day (due to stock changes in
your product list, new products, etc...), do you need to post your
google sitemap that much?

I mean, if you generate and upload one sitemap, just at the launch of
the website, does google still crawles your site, or does it just look
at the uploaded sitemap?

Thanks for any help

Steven